Please refer to the following URL to know the differences between DLL and EXE,http://onlydifferencefaqs.blogspot.in/2012/07/net-framework-difference-faqs-1_11.html

DLL:Dynamic Link library
1)it is inprocess Componet
2)it can be used more than one.
3)it can be  created when ever you take class library project.
EXE:Excutable file
1)it is Out Processcomponet.
2)It can be Used only once.
3)It can be created when ever you take Console Application or Other

Dll is a passive component which doesnot have ownership, whereas Exe is an active component which has ownership. Dll is always loaded by Exe.

DLL is the InProcess and EXE is the Out-of-Process component.

DLL runs in the Same Memory space as Calling Application where Exe runs on there own Memory Space.

DLLs can be used as the COM Component and Deploy in COM+ Services.

following r the few differences b/w dll and exe:

*exe is selfexecutable where as dll is not self executable.

*dll provides reusability where as exe dont.

*exe contains main function, where as dll doesnt contains any main function.